If I arbitrarily set an GameObject's transform to Vector3(x,y,z) are there any limits to x,y,z ?

There is no mathematical limit per se, but x y and z are floating point numbers, so they are technically limited by the highest and lowest numbers that are representable with 32 bit floating points.

The "bounding box" of the Unity space should therefore consist of a cube with corners at (float.MinValue, float.MinValue, float.MinValue) and (float.MaxValue,float.MaxValue,float.MaxValue).

The max and min are plus/minus 3.402823E+38.

If it works like any other 3D programme, then there isnt a hard limit on how far you can get away from the world 0 point. But, your posistion becomes more and more inaccurate the further you come away, making movement jittery and unprecise.

I'm not sure, but more relevant is that there is a limit to where the scene views camera can go. So you won't be able to edit/see objects that are to far away.